1. Field of the Invention
This invention relates to volatile divalent metal alkoxides useful for the CVD deposition of metal oxides to a substrate. More particularly, this invention relates to volatile alkoxides of barium, strontium, or calcium used to deposit oxides used to make thin, superconducting copper oxide films.
2. Description of the Prior Art
The utility of copper oxides of barium, strontium, or calcium are well known. Difficulty exists in preparing uniform, consistent, thin films of these superconducting materials. By uniform is meant both uniform in thickness of the layer of material deposited and uniform in the chemical makeup of the layer deposited. By consistent is meant that the uniformity of the film is consistent for each run or application of a coating layer.
Chemical vapor deposition (CVD) is a well known method used in the optical fiber and semiconductor field for depositing uniform and consistent films of material. A CVD process requires volatile precursors for the oxides of the thin film. This has been a problem in making superconducting films of superconducting materials containing barium, strontium, or calcium because few volatile precursors of these oxides are known or suggested for this purpose.
Bunker et al., in U.S. Pat. No. 4,839,339, describes a precipitation method of making superconductor precursor mixtures. In the description of the state of the art, Bunker et al. notes that many methods of preparing superconductor mixed-oxides have been tried including CVD. Bunker avoids CVD and describes another process.
Metal Alkoxides of the superconducting elements have been used to make superconductors, particularly the copper oxides including Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7. The metal alkoxides are used in a solution or in gel form as described by Frahrenholtz et al., Preparation of YBa.sub.2 Cu.sub.3 O.sub.7 .delta. from Homogeneous Metal Alkoxide Solution, pp. 141-147; and Laine et al., Organometallic Precursors for the Fabrication of Hioh T.sub.c Superconducting Fibers, pp. 450-455, both published in Research Update, 1988, CERAMIC SUPERCONDUCTORS II, Edited by Man F. Yan, American Ceramics Society, Inc, Westervill, Ohio; G. Moore et al., Sol-Gel Processing of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x Using Alkoxide Precursors: Two Systems Yielding High Degree of Thin Film Orientation and Crystal Growth, MATERIALS LETTERS, Vol 7, No. 12, pp. 415-424, March 1989; Horowitz et al, Submicrometer Superconducting YBa.sub.2 Cu.sub.3 O.sub.6+x Particles Made by a Low-Temperature Synthetic Route, SCIENCE, Vol. 243, pp. 66-69, Jan. 6, 1989, to prepare the copper oxide of the superconducting composition. At page 141, Fahrenholtz noted the problems with preparing superconductor thin films of the copper oxides.
Berry et al. reported making superconductor films by CVD. See Berry, A. D.; Gaskill, D. K.; Holm, R. T.; Cukauskas, E. J.; Kaplan, R.; Henry, R. L. Appl. Phys. Lett. 52(20), pp. 1743 (1988). Yamane et al. have also reported the preparation of YBa.sub.2 Cu.sub.3 O.sub.7-x films by CVD. The volatile precursor are metal chelates. See Yamane et al., Preparation of YBa.sub.2 Cu.sub.3 O.sub.7-x Films by Chemical Vapor Deposition, CHEMISTRY LETTERS, pp. 939-940, 1988. Yamane et al. also reports the preparation of BiSrCaCuO films by CVD. See Yamane at al., Preparation of Bi--Sr--Ca--Cu--0 Films by Chemical Vapor Deposition with Metal chelate and Alkoxide, CHEMISTRY LETTERS, PP. 1515-1516, 1988. Shinohara et al. has reported the CVD of superconducting Y-Ba-Cu-O using a fluorine containing precursors. Initially, fluoride films are deposited and subsequently converted to the oxide with water vapor. See Shinohara, K.; Munahata, F.; Yamanaha, M., Japn. J. Appl. Phys. 27(9), L1683, (1988).
Studies have been made of the alkoxides, but there is a tendency for the compounds to be involatile and insoluble as reported by Adams et al., Magnetism, Electronic Spectra, and Structure of Transition Metal Alkoxides, Aust. J. Chem, Vol 19, pp. 207-10, 1966. Volatile Cu I alkoxides are known. Cuprous tert-Butoxide. J. Am. Chem. Soc., Vol. 94:2, pp. 658-659, Jan. 26, 1972. Some volatile double ethoxides are known. Govil et al., Some Double Ethoxides of Alkaline Earth Metals With Aluminum, SYN. REACT. INORG. METAL-ORG. CHEM., Vol. 5(4), pp. 267-277 (1975).
In U.S. Pat. No. 4,717,584, Aoki et al. reports the preparation of magnetic thin films by plasma CVD using metal alkoxides including diethoxy, dipropoxy and dibutoxy barium and strontium. In the Aoki process, argon is bubbled through an alcohol solution of the alkoxides. The stream of argon, presumably saturated with alkoxide and alcohol, is conducted to a substrate where the alkoxide is deposited. A plasma is used to decompose the alkoxides to magnetic oxides.
As reported in the text METAL ALKOXIDES, Bradley et al., Academic Press New York, 1978, at pages 46-50, the alkoxides of most metals and the alkali metals and the Group II metals, such as Ba in particular, are not volatile or are sparingly volatile. Often, these alkoxides decompose instead of subliming. Some of the alkoxides are volatile such as NaOBu.sup.t, Na and K fluorinated alkoxides, but writers in the field expected the stability of the alkoxides to decrease as the atomic weight of the metal atom increases. See Dear et al., VOLATILE FLUORINATED ALKOXIDES OF THE ALKALI METALS, Inorganic Chemistry, Vol 9, pp. 2590-2591, (1970) at p. 2591. A problem exists in providing a volatile precursor for barium, calcium and strontium which provides uniform, consistent films of superconducting films on substrates.